Retention and drainage are important properties of a papermaking process that papermakers are always seeking to optimize.
One method of improving the retention of cellulosic fines, mineral fillers and other furnish components on the fiber mat is the use of a coagulant/flocculant system, added ahead of the paper machine. To use such a system, a papermaking slurry (or furnish) is created out of a pulp. To this slurry is added a coagulant, with said coagulant being selected from the group consisting of low molecular weight cationic synthetic polymers, starch and alum. The coagulant generally reduces the negative surface charges present on the particles in the slurry, particularly cellulosic fines and mineral fillers, and thereby accomplishes a degree of agglomeration of such particles. The next item added is a flocculant. Flocculants typically are high molecular weight anionic synthetic polymers which bridge the particles and/or agglomerates, from one surface to another, binding the particles into large agglomerates. The presence of such large agglomerates in the slurry as the fiber mat of the paper sheet is being formed increases retention.
While a flocculated agglomerate usually does not interfere with the drainage of the fiber mat to the extent that would occur if the furnish were gelled or contained an amount of gelatinous material, there is a noticeable reduction in drainage efficiency when such flocculated agglomerates are filtered by the fiber web, because the pores thereof are to a degree reduced. Hence, retention usually is increased with some degree of deleterious effect on the drainage.
Another system employed to provide an improved combination of retention and drainage (or dewatering as it is sometime known) is described in U.S. Pat. Nos. 4,753,710 and 4,913,775, the disclosures of both of these patents being incorporated herein by reference. In brief, such method first adds to the aqueous cellulosic papermaking suspension a high molecular weight linear cationic polymer before shearing the suspension, followed by the addition of bentonite after shearing. The shearing generally is provided by one or more of the cleaning, mixing and pumping stages of the papermaking process. The shearing breaks down the large flocs formed by the high molecular weight polymer into microflocs. Further agglomeration then ensues with the addition of the bentonite clay particles.
Another system uses the combination of cationic starch followed by dispersed silica to increase the amount of material retained on the web by the method of charge neutralization and adsorption of smaller agglomerates. This system is described in U.S. Pat. No. 4,388,150, inventors Sunden et al., issued Jun. 14, 1983.
In another system, a high molecular weight cationic polymer is added to the slurry before shearing. Then an organic microparticle is added to the slurry after the introduction of shear. The organic microparticle is a medium molecular weight anionic polymer such as the copolymers of acrylic acid described in U.S. Pat. No. 5,098,520, the disclosure of which is incorporated herein by reference. Or the organic microparticle can be a medium molecular weight anionic sulfonated polymers such as those described in U.S. Pat. No. 5,185,062, the disclosure of which is herein incorporated by reference.
There continues to be a need to identify new additive or additives that when added in specific combinations result in improvement in retention and drainage in a papermaking process.